This invention relates to a printing plate, a method of making such a printing plate, and a method of printing using such a plate to form a desired image on a medium. More particularly, the printing plate of this invention employs a fluid composition comprising a compound which comprises a nitrogen-containing heterocyclic moiety that is acid-neutralized to adhere to a substrate that has a plurality of basic sites. The fluid composition is applied by ink jetting to a substrate, providing a printing plate that is ready-to-use on a press without having to develop it.
The offset lithographic printing process has long used a developed planographic printing plate having oleophilic image areas and hydrophilic non-image areas. The plate is commonly dampened before or during inking with an oil-based ink composition. The dampening process utilizes an aqueous fountain solution such as those described in U.S. Pat. Nos. 3,877,372, 4,278,467 and 4,854,969. When water is applied to the plate, the water will form a film on the hydrophilic areas, which are the non-image areas of the plate, but will contract into tiny droplets on the oleophilic plate areas, which are the image areas. When a roller carrying an oil-based ink composition is passed over the dampened plate, it will not ink the non-image areas that are covered by the aqueous film, but will emulsify the water droplets on the water repellant image areas, which will then take up ink. The resulting ink image is transferred, or xe2x80x9coffset,xe2x80x9d onto a rubber blanket, which is then used to print onto a medium such as paper.
It has been proposed to apply xe2x80x9cdirectxe2x80x9d ink jet printing techniques to lithographic printing. For example, European Patent Publication No. 503,621 discloses a direct method to make lithographic plates by jetting a photocurable ink onto the plate substrate, and then exposing the plate to ultraviolet radiation to harden the image area. An oil-based ink may then be transferred to the image area for printing onto a printing medium. But, neither the resolution of ink drops jetted onto the substrate, nor the durability of the lithographic printing plate with respect to printing runlength was disclosed.
It has also been proposed to apply the direct ink jet printing techniques without the additional steps of chemical development of the plate. This approach advantageously results in lower production costs and a more environmentally acceptable printing process. However, in such techniques it is difficult to control the spreading of the ink-jetted fluid that forms the oleophilic ink-accepting regions on the printing plate substrate. Such xe2x80x9cdot spreadingxe2x80x9d causes lower printing image resolution and reduced image quality. For example, European Patent Application No. 591,916 A2 discloses a water-based ink having a polymer containing anhydride groups which are thermally cross-linked with a hydroxy-functional polymer. This formulation is applied by jetting the formulation which is at room temperature onto a room temperature substrate. However, this formulation does not achieve good control of dot spreading.
U.S. Pat. No. 4,833,486 discloses the apparatus and process for imaging a plate with a xe2x80x9chot meltxe2x80x9d type of inkjet printer. The image is produced by jetting at high temperature a xe2x80x9cphase changexe2x80x9d type of ink which solidifies when it hits the cooler substrate. The ink becomes instantaneously solid rather than remaining a liquid or gel which is thereafter cured to form a solid. However, such an ink does not provide good resistance to press run due to the wax-type nature of the ink formulation.
U.S. Pat. No. 5,942,335 discloses the use of a polymer containing a nitrogen-containing heterocyclic group, namely a polymer of 4-vinyl pyridine, in the formulation of an ink receiving layer of an ink jet recording sheet. However, the use of such a compound in a fluid composition applied directly to a printing plate substrate to form an imaged, ink-receptive layer is not disclosed.
Thus, it would be advantageous to employ a printing plate capable of extended press run length which does not require chemical development.
It is one object of this invention to provide such a printing plate. It is another object of this invention to provide a method of preparing such a printing plate. It is yet another object of this invention to provide a method of using such a printing plate. The printing plate of this invention may advantageously be prepared without a chemical development step typically required. The printing plate of this invention is also capable of extended press run length.
The fluid composition of this invention is suitable for ink jetting upon a substrate and comprises a compound that comprises at least one nitrogen-containing heterocyclic moiety, wherein the nitrogen-containing heterocyclic moieties of the compound are at least partially neutralized with an acid, in aqueous solution.
The printing plate of this invention is prepared by: (a) providing a substrate; and (b) applying by ink jetting to the substrate a fluid composition as described. In a preferred embodiment, the compound that comprises at least one nitrogen-containing heterocyclic moiety is selected from the group consisting of polymers or copolymers of 2-vinylpyridines, polymers or copolymers of 4-vinylpyridines, polymers or copolymers of ethylimidazolidone methacrylates, 2-pyridyl ethyl trimethoxysilanes, and mixtures thereof. In particularly preferred embodiments, the nitrogen-containing heterocyclic moieties of the compound are partially or fully neutralized with formic acid.
The printing plate of this invention is capable of extended press run length and advantageously avoids the need of chemical development.
To achieve extended printing runs, the oleophilic material must adhere well to the substrate. Adhesion of the oleophilic material may be controlled in at least two ways. First, the oleophilic material should have a chemical interaction with the substrate that provides a type of chemical binding and promotes adhesion. For example, the chemical composition of the oleophilic material can be varied to promote its adhesion to the substrate. Also, the composition of the substrate can be varied to increase binding of the oleophilic material. Second, the substrate should provide microscopic topology that allows the oleophilic material to interlock mechanically with the substrate when dry or hardened. Mechanical interlocking can be affected by roughening the surface of the substrate. Thus, by controlling these variables, a printing plate can be made with increased adhesion of the oleophilic material, and correspondingly longer printing run operation.
In the invention described here, the oleophilic material is placed on the substrate by ink jetting a fluid composition comprising the oleophilic material. Because the fluid composition is the vehicle to carry the oleophilic material, the wettability of the substrate by the ink-jetted droplets of fluid composition and the spreading of the ink-jetted droplets are properties that affect resolution of the printing process. Too much spreading of the ink-jetted droplets of fluid composition will reduce printing resolution. The substrates used here typically have high surface tension and allow aqueous fluids to spread completely and rapidly over their surface. While this benefits adhesion of the oleophilic material to the substrate, it disadvantageously reduces image resolution in printing. Here, the fluid composition is prepared to balance these properties, and provide good adhesion for longer press runs in conjunction with high image resolution.
The spreading of droplets is reduced in two ways by the present invention. First, the fluid composition comprising the oleophilic material is interfacially matched to the substrate, as described below. Second, by pretreating the substrate surface with a surfactant to lower its surface tension, the wetting and spreading of droplets is reduced. Thus, by these and other features inherent in the composition and method described here, excellent resolution can be obtained, as well as long-lasting adhesion of the dried oleophilic material to the substrate.
The printing plate of this invention encompasses lithographic printing plates, flexographic printing plates, and gravure printing plates.
Conventional printing plate substrates such as aluminum, polymeric film, and paper may be used as the printing plate substrate of this invention. The printing plate substrate may be subjected to treatments such as electrograining, anodization, and silication to enhance its surface characteristics. The surface characteristics that are modified by such treatments are roughness, topology, and the nature and quantity of surface chemical sites. In a preferred embodiment the printing plate surface comprises a plurality of basic sites, such as, but not limited to, sodium silicate groups.
Substrates that can be employed are given in Table 1. Substrates chosen for use in this invention are preferably based on aluminum oxide, and may be subjected to various conventional surface treatments as are well known to those skilled in the art to give a surface that has a plurality of basic sites in the Bronsted acid-base view. These treatments also result in different surface roughness, topology, and surface chemical sites, as summarized in Table 1.
xe2x80x9cAAxe2x80x9d means xe2x80x9cas anodized.xe2x80x9d The aluminum surface is first quartz grained and then anodized using DC current of about 8 A/cm2 for 30 seconds in a H2SO4 solution (280 g/liter) at 30xc2x0 C.
xe2x80x9cEGxe2x80x9d means xe2x80x9celectrolytic graining.xe2x80x9d The aluminum surface is first degreased, etched and subjected to a desmut step (removal of reaction products of aluminum and the etchant). The plate is then electrolytically grained using an AC current of 30-60 A/cm2 in a hydrochloric acid solution (10 g/liter) for 30 seconds at 25xc2x0 C., followed by a post-etching alkaline wash and a desmut step. The grained plate is then anodized using DC current of about 8 A/cm2 for 30 seconds in a H2SO4 solution (280 g/liter) at 30xc2x0 C.
xe2x80x9cPVPAxe2x80x9d is a polyvinylphosphonic acid. The plate is immersed in a PVPA solution and then washed with deionized water and dried at room temperature.
xe2x80x9cDSxe2x80x9d means xe2x80x9cdouble sided smooth.xe2x80x9d The aluminum oxide plate is first degreased, etched or chemically grained, and subjected to a desmut step. The smooth plate is then anodized.
xe2x80x9cSilxe2x80x9d means the anodized plate is immersed in a sodium silicate solution to coat it with an interlayer. The coated plate is then rinsed with deionized water and dried at room temperature.
xe2x80x9cPGxe2x80x9d means xe2x80x9cpumice grained.xe2x80x9d The aluminum surface is first degreased, etched and subjected to a desmut step. The plate is then mechanically grained by subjecting it to a 30% pumice slurry at 30xc2x0 C., followed by a post-etching step and a desmut step. The grained plate is then anodized using DC current of about 8 A/cm2 for 30 seconds in an H2SO4 solution (280 g/liter) at 30xc2x0 C. The anodized plate is then coated with an interlayer.
xe2x80x9cG20xe2x80x9d is a printing plate substrate which is described in U.S. Pat. No. 5,368,974, the disclosure of which is incorporated herein by reference in its entirety.
xe2x80x9cCHBxe2x80x9d means chemical graining in a basic solution. After an aluminum substrate is subjected to a matte finishing process, a solution of 50 to 100 g/liter NaOH is used during graining at 50 to 70xc2x0 C. for 1 minute. The grained plate is then anodized using DC current of about 8 A/cm2 for 30 seconds in an H2SO4 solution (280 g/liter) at 30xc2x0 C. The anodized plate is then coated with a silicated interlayer.
xe2x80x9cPFxe2x80x9d substrate has a phosphate fluoride interlayer. The process solution contains sodium dihydrogen phosphate and sodium fluoride. The anodized substrate is treated in the solution at 70xc2x0 C. for a dwell time of 60 seconds, followed by a water rinse, and drying. The deposited dihydrogen phosphate is about 500 mg/m2.
A xe2x80x9cbasicxe2x80x9d surface will have a plurality of basic sites and acidic sites present, with the basic sites predominating to some degree. Similarly, an xe2x80x9cacidicxe2x80x9d surface will have a plurality of acidic sites and basic sites present, with the acidic sites predominating to some degree. It is known by one of ordinary skill in the art that the PG-Sil printing plate substrate appears to have a higher silicate site density than the DS-Sil printing plate substrate, and is more basic. It is also known that the G20 printing plate substrate exhibits less acidic character than AA printing plate substrates.
In preferred embodiments, the oleophilic compound of the present invention that forms the ink-receiving layer may be prepared from a starting polymer that comprises base groups by partially or filly neutralizing the base groups with acid to give a product polymer that comprises the base groups and conjugate acid groups, or comprises solely the conjugate acid groups. The oleophilic product polymer is used to make long-lasting printing plates by enhancing the adhesion of the product polymer to the substrate. The conjugate acid groups of the product polymer react with, for example, the silicated sites, xe2x80x94SiOxe2x88x92Na+, of the basic substrate in ionic double exchange to bind the polymer to the substrate. This chemical binding works in combination with the physico-chemical adsorption of the product polymer to the roughened substrate to provide strong adhesion of the ink-receiving layer, a more durable printing plate, and longer printing press runs.
The starting polymer of preferred embodiments comprises a monomer that comprises a nitrogen-containing heterocyclic moiety. The starting polymer can be a homopolymer, copolymer, terpolymer, and the like. By xe2x80x9ccopolymerxe2x80x9d we mean any polymer comprised of more than one type of monomer, prepared in a copolymerization. By xe2x80x9cterpolymerxe2x80x9d we mean a polymer consisting essentially of three types of monomers, prepared in a copolymerization. Thus, a copolymer can include a terpolymer.
In preferred embodiments, the oleophilic compound of the present invention that forms the ink-receiving layer may also be prepared from a starting monomeric compound that comprises base groups, such as a substituted silane.
A starting polymer of this invention preferably has a surface energy, as measured according to the Owens-Wendt method, as described in J. Applied Polymer Science, 1969, Vol. 13, pp. 1741, based on two liquid probes, water and diiodomethane, of less than about 50 dynes/cm.
The starting polymer most preferably comprises a monomer selected from the group consisting of 2-vinylpyridines, 4-vinylpyridines, ethylimidazolidone methacrylates, and mixtures thereof. These polymers comprise a nitrogen-containing heterocyclic moiety that can react with formic acid to produce conjugate acid groups, XNH+HCOOxe2x88x92, where XN is a pyridyl or uridyl group, to react with the substrate.
In preferred embodiments, the oleophilic compound of the present invention that forms the ink-receiving layer is a compound which comprises at least one nitrogen-containing heterocyclic moiety, and the nitrogen-containing heterocyclic moieties are at least partially neutralized with an acid, resulting in excellent adhesion to the substrate. An illustrative example of the oleophilic compound of the present invention that forms the ink-receiving layer is 2-pyridyl ethyl trimethoxysilanes, and the like.
Acids suitable for neutralizing the base groups of the oleophilic compound conform to one of the formulae in the group consisting of H(CH2)nCOOH, and HOCHRCOOH, where R is xe2x80x94H, xe2x80x94CH3, or xe2x80x94CH2CH3, and n is from zero to six. Acids used in preferred embodiments are formic, acetic, lactic, and glycolic, while formic acid is especially preferred.
The ink-receptive layer produced with the oleophilic compound has excellent adhesion to the substrate surface, and as set forth in further detail below, the resulting printing plate exhibits extended press run length. Advantageously, the superior results of the printing plate of this invention are achieved without chemical development.
The fluid composition comprising the oleophilic compound is preferably applied by ink jetting to the substrate surface, typically by an ink jet printer using equipment and techniques which are well known to those skilled in the art. In this manner, the substrate is imaged so that after the fluid composition dries, an ink receptive layer is formed in the desired image on the surface of the substrate.
Without intending to be bound by any one particular theory, the nature of the oleophilic compound enhances the resolution achieved in printing because the fluid composition can be xe2x80x9cinterfacially matchedxe2x80x9d to the substrate. By xe2x80x9cinterfacial matchingxe2x80x9d we mean providing an oleophilic compound comprising conjugate acid groups to be used with a basic substrate, or conversely, providing an oleophilic compound comprising conjugate base groups to be used with an acidic substrate. The droplet of fluid composition on the substrate spreads as a primary film in advance of the bulk of the droplet. Further, water and formic acid in the advancing primary film evaporate relatively quickly, leaving behind an increased concentration of neutral base groups on the oleophilic compound. These neutral base groups exhibit repulsion from the basic sites of the substrate, and slow the spreading of the droplets of fluid composition on the substrate surface. At the same time, in the bulk of the droplet, an excess of conjugate acid groups on the neutralized compound continue to give good adhesion to the substrate. Thus, reduced droplet spreading is achieved with good adhesion of the oleophilic compound to the silicated plate.
In preferred embodiments, the fluid composition contains a humectant to help retain water in the ink jet nozzle during idle periods. Suitable humectants include glycerol, aliphatic glycols, aliphatic glycol ethers, and mixtures thereof. The fluid composition may also contain other additives, such as colorants, biocides, corrosion inhibitors, and anti-foam agents, as used by those of skill in the art of ink jet printing, without loss of the characteristic properties of this invention.
Adsorbing a surfactant to a conventional printing plate substrate, prior to application of an ink receptive layer, can improve the image resolution achieved. Such a surfactant-pretreated substrate will be termed a xe2x80x9cprinting plate precursorxe2x80x9d herein. A printing plate may be prepared from the printing plate precursor by imagewise applying a fluid composition as described above to the substrate. In a preferred embodiment, the fluid composition is applied by means of an ink jet printer, and then dried to form an ink receptive layer in the form of the desired image. Advantageously, chemical development of the printing plate is not required.
Adhesion of the oleophilic compound from the fluid composition to the substrate after drying is not diminished substantially by the presence of the precursor plate surfactant, which tends only to slow the spreading of the aqueous droplet deposited by the ink jet nozzle. Thus, the precursor plate surfactant can increase resolution without reducing press run length. Surfactants that can be used for the precursor include alkyl tail surfactants, fluorosurfactants and siliconated surfactants.
Illustrative examples of alkyl tail surfactants include sodium dodecylsulfate, isopropylamine salts of an alkylarylsulfonate, sodium dioctyl succinate, sodium methyl cocoyl taurate, dodecylbenzene sulfonate, alkyl ether phosphoric acid, N-dodecylamine, dicocoamine, 1-aminoethyl-2-alkylimidazoline, 1-hydroxyethyl-2-alkylimidazoline, and cocoalkyl trimethyl quaternary ammonium chloride, polyethylene tridecyl ether phosphate, and the like.
Illustrative examples of fluorosurfactants useful in preferred embodiments of the present invention and their commercial trade names are set forth in Table 2.
(1) FSJ also contains a nonfluorinated surfactant.
(2) R ** contains an ammonium function.
(3) y or z=0 to about 25.
ZONYL surfactants are commercially available from E.I. du Pont de Nemours and Co. and have a distribution of perfluoroalkyl chain length. FLUORAD surfactants are commercially available from 3M Company and have a narrow distribution of the hydrophobic chain length.
Illustrative siliconated surfactants include the following non-exhaustive listing: polyether modified poly-dimethyl-siloxane, silicone glycol, polyether modified dimethyl-polysiloxane copolymer, and polyether-polyester modified hydroxy functional polydimethyl-siloxane.
The precursor plate surfactant may be adsorbed onto the substrate by any conventional method, preferably by immersion of the substrate in an aqueous solution of the surfactant for a time, typically one minute, which is effective to permit adsorption of the surfactant upon the substrate. In a particularly preferred embodiment, any non-adsorbed surfactant is then removed from the printing plate substrate surface. Preferably, the substrate is rinsed with water to remove non-adsorbed surfactant, then dried. The resulting printing plate precursor has a surfactant on at least one surface, in an amount effective to improve the resolution of printing.
An imaged substrate prepared by imagewise applying a fluid composition to a substrate could also be used, for example, as a precursor for a printed circuit board in which conductive metals are deposited onto the imaged substrate.
The following examples are given to illustrate preferred embodiments of the present invention and are not intended to limit the invention in any way. It should be understood that the present invention is not limited to the above-mentioned embodiments. Numerous modifications can be made by one skilled in the art having the benefits of the teachings given here. Such modifications should be taken as being encompassed within the scope of the present invention as set forth in the appended claims.